This invention relates generally to the field of nondestructive testing, and more particularly to a method and apparatus for measuring the distribution of mass density of materials, especially paper and paper board, using electron beams.
Paper is made of a great number of fibers and the uniformity of the fiber distribution is a very important factor which determines the paper's properties such as strength, smoothness, and printability. The uniformity of the fiber distribution is determined while the fiber suspension is delivered to the drainage section of the paper machine or in the drainage section itself. The uniformity of the fiber distribution is called "formation" and is usually evaluated by its "look-through" appearance when held up to the light. A variety of terms such as "flocced", "cloudy", "uneven", "uniform", etc. are used to describe formation
Since formation is usually evaluated by visual perception, the evaluation is not absolute or objective. Many attempts have been made to measure the uniformity of fiber distribution objectively. An early scanning device based on the transmission of light through a rotating disc of paper can be found in U.S. Pat. No. 1,991,599 of Davis et al. Many variations of this original design have been proposed. Among the most important ones are the Thwing-Albert test method and the Quebec North Shore Mead (QNSM) tester. In the mill, on-line formation meters based on light transmission such as described in U.S. Pat. No. 4,644,174 of Ouellette et al. and U.S. Pat. No. 4,707,223 of Sabater et al. are being used to assess formation in the paper.
However, light transmission in paper is affected by many factors such as the degree of bonding, pressing and the presence of scattering fillers. Other methods for assessing mass variations rather than opacity variations should therefore be used to determine the fiber distribution within the sheet. Herdman and Corte in volume 81 of the Pulp and Paper Canada, P.T261, October, 1980, introduced the distribution of mass density approach. In their work, beta-radiation was used in place of visible light. Since beta-rays are attenuated and absorbed in matter, their absorption is related to the amount of mass being traversed. The density of mass distribution is therefore a true basis weight representation.
In laboratories, the distribution of mass density of paper may be obtained by measuring the attenuation of beta-particles directly as a strip of paper is scanned as presented by Balodis (Appita, Vol. 18, page 184, May, 1965). This is a slow and tedious operation, especially if the source is weak and/or very small areas need to be scanned. A much quicker way is to obtain a beta-radiograph of paper as presented by Attwood and Parker ("Basis Weight Variations Over Small Areas of Paper", Paper Technology, Vol. 3, P. 435, May, 1962). This is done by sandwiching a paper sample between a radioactive plastic .sup.14 C source and a photographic film. On developing the film with a suitable calibration, the distribution of mass density can be calculated from optical density measurements. This method may take 4 to 24 hours and the distribution of mass density of thick paper samples (&gt;150 g/m.sup.2) cannot be determined by this method.
In the mill, beta-gages described as in U.S. Pat. No. 3,413,192 of Beecher and U.K. Pat. No. 1,271,438 are being used for measuring the distribution of mass density in paper. This system usually measures larger areas as large as 25 cm.sup.2 and gives the average and the variation of the basis weight, the unit of which is g/m.sup.2, but does not give the variation in smaller areas of the sample. Since these techniques use open beta-radiation sources, it can be dangerous for the operators to expose themselves to the radiation. Therefore, another method to measure the distribution of mass density, which is rapid, safe, accurate, applicable to a wide range of basis weight and yet capable of detecting small scale variations, would be a significal advantage in both industrial and laboratory settings.
In the field of electron microscopy, a technique to measure the thickness or the density of a very thin sample by Transmission Electron Microscopy was developed by Marton and Schiff ("Determination of Object Thickness in Electron Microscopy", Journal of Applied Physics, Vol. 12, P. 759, October, 1941). This technique has been used to determine the thickness and the density of biological samples and synthetic lattices placed in a conventional sample chamber at the focal plane of a transmission electron microscope, with a magnified image of the sample being analyzed. However, according to Zeitler and Bahr, ("A Photometric Procedure for Weight Determination of Submicroscopic Particles Quantitative Electron Microscopy", Journal of Applied Physics, Vol. 33, P. 847, March, 1962), this technique has a limitation in sample thickness. Only a sample which is less than 0.4 microns in thickness can be measured by this method. Paper samples are far thicker than this and cannot be measured by this method.